The present invention relates to stable paving compositions having improved low and high temperature properties. More particularly, the invention is concerned with mixtures of an asphaltic material and solid particles encapsulated with a cross-linked polymer, the encapsulated particles being uniformly dispersed in the asphaltic material.
Mixtures of polymers and solid particles in asphalt are always subjected to destabilizing forces such as gravity force and attraction forces between the dispersed particles. High molecular weight polymers are never completely soluble in asphalt, resulting in the formation of a polymer dispersed phase. The coalescence of fine particles followed by creaming is the main mechanism that is favored by decreasing surface energy of polymer particles and gravity force. The final result is formation of a thin layer of polymer on the top of the asphalt. Reducing the size of polymer particles is a remedy to extensively decrease the rate of coalescence, but creaming and Brownian motion still exist and can cause coalescence of up going polymer particles. The concept of steric stabilization is very limited to finding a completely soluble polymer in asphalt ingredients and reactive chemical functional groups in asphalt.
Carbon black has already been used as a modifier for paving grade asphalt. However, suspension of high density particles in asphalt undergo precipitation during long periods of storage at elevated temperatures (160xc2x0 C.).
It is therefore an object of the present invention to overcome the above drawbacks and to provide stable paving compositions which comprise mixtures of an asphaltic material and solid particles, and which exhibit no phase separation between the solid particles and the asphalting material at elevated temperatures or during long periods of storage.
According to the invention, there is provided a paving composition comprising a mixture of about 90 to 99.5 wt. % of an asphaltic material and about 0.5 to 10 wt. % of solid particles each encapsulated with a cross-linked polymer, the encapsulated particles being uniformly dispersed in the asphaltic material. The encapsulated particles have a particle-to-polymer weight ratio so as to provide a density match between the encapsulated particles and the asphaltic material.
Applicant has found quite unexpectedly that by controlling the weight ratio of the solid particles to the polymer encapsulating same so that the density of the encapsulated particles is approximately equal to the density of the asphaltic material, there is no phase separation between the encapsulated particles and the asphaltic material at elevated temperatures and/or during long periods of storage. There is also no separation between the solid particles and the polymer encapsulating same due to the cross-linking of the polymer. Care should be taken to avoid exceeding about 10 wt. % of polymer since above such a concentration there is a phase inversion as the polymer swells and the asphaltic material becomes dispersed in the swollen polymer.
According to a preferred embodiment, the asphaltic material and encapsulated particles are present in amounts of about 95-97 wt. % and about 3-5 wt. %, respectively. Preferably, the weight ratio of the solid particles to the polymer is about 3:1.
Examples of suitable solid particles which may be used include fine powders of carbon black, silicate or kaolin. Carbon black having a particle size of about 10xcexc is preferred.
Examples of suitable polymers which may be used for encapsulating the solid particles include polyolefins such as polyethylene, polypropylene and polystyrene, and polydienes such as polybutadiene and polyisoprene.
The term xe2x80x9casphaltic materialxe2x80x9d as used herein is meant to include petroleum asphalts, also called oil asphalts, such as:
1) straight-reduced asphalt by:
a) atmospheric, vacuum or partial-vacuum distillation,
b) solvent precipitation;
2) thermal asphalts, such as residues form cracking operations in petroleum stocks; and
3) air-blown asphalts:
a) straight-blown without catalysts,
b) blown in the presence of catalysts.
According to a particularly preferred embodiment, the encapsulated particles are obtained by catalytically grafting the polymer on the solid particles. Preferably, the polymer is catalytically synthesized on the surface of the particles and the encapsulated particles are treated with a cross-linking agent to cause cross-linking of the polymer. Examples of suitable cross-linking agents include peroxides such as dicumyl peroxide.
The paving compositions according to the invention have a higher complex viscosity at high temperatures (60xc2x0 C.) and are more ductile at low temperatures (xe2x88x9215xc2x0 C.) in comparison with the base (unmodified) asphaltic material. Moreover, the compositions are completely stable at elevated temperatures (160xc2x0 C.) or during long periods of storage.